1. Field of the Invention
The present invention relates to a method for evaluating the magnetized state of a pinned layer of a giant magnetoresistive (GMR) spin-valve element, a method for manufacturing a magnetic head with a GMR spin-valve element, and a magnetic storage device.
2. Description of the Related Art
Along with the increase of recording density of magnetic storage devices, use of a so-called spin-valve element or GMR element has been spreading, which element is used as a reproducing element to reproduce information from a magnetic recording medium. As illustrated in FIG. 1, a spin valve element 100 includes a spin-valve (SV) film 110 consisting of an antiferromagnetic layer 112, a pinned layer (fixed magnetized layer) 113, a nonmagnetic interlayer 114, and a free layer (or magnetic free layer) 115. The direction of magnetization of the pinned layer 113 is fixed in the Z2 direction by mutual interaction with the antiferromagnetic layer 112. The magnetization of the free layer 115 points in the Y1 direction, which is perpendicular to the magnetization MPIN of the pinned layer 113 when no magnetic field Hsig is applied from the magnetic recording medium 117. In the GMR spin-valve element 100, sense current Is is supplied to the SV film 110 in the Y1 direction, which direction is parallel to the surface of the magnetic recording medium 117, and a change in electric resistance caused by change of the angle between the magnetization MFR of the free layer 115 and the magnetization MPIN of the pinned layer 113 is detected as a voltage change. When the magnetic field Hsig generated from the magnetic recording medium 117 is applied in the Z1 direction or Z2 direction so as to affect the magnetization MFR of the free layer 115, the magnetization MFR changes. In other words, the GMR spin-valve element 100 reproduces the information from the magnetic recording medium 117 using a certain level of sense current Is flowing through the SV film 110 to monitor a voltage change corresponding to the change in electric resistance.
The magnetization MPIN of the pinned layer 113 is fixed through the mutual interaction with the antiferromagnetic layer 112; however, if an excessive amount of electric current happens to flow in the SV film 110 due to, for example, electrostatic discharge, the magnetization MPIN may be inverted. If the magnetization MPIN is inverted, the polarity of the reproduced output signal is reversed with respect to the magnetic field from the magnetic recording medium 117. This phenomenon is illustrated in FIG. 2A and FIG. 2B. If the magnetization of MPIN of the pinned layer 113 is inverted, the polarity of the reproduced waveform is inverted, as illustrated in FIG. 2B, with respect to that of the normal SV film of a GMR element shown in FIG. 2A. If this happens, a read error will occur. In addition, if the magnetization MPIN of the pinned layer 113 deviates from a correct direction, the symmetry of the positive and negative wave heights of the reproduced signal is degraded, and read error increases.
A conventionally proposed evaluation method is to vary the level of the sense current and to measure a level at which the magnetization of the pinned layer 113 is inverted. The measurement result is used as one of performance evaluation factors of the GMR element 100. With the conventional method, the sense current Is is supplied in a direction opposite to the ordinary current flow, so as to apply the magnetic field produced by the sense current in the Z1 direction, which direction is opposite to the magnetization MPIN of the pinned layer 113. As the sense current Is increases, the intensity of the magnetic field increases, and the magnetization of the pinned layer 113 cannot resist this magnetic field any longer. Finally, the magnetization of the pinned layer 113 inverts to the direction of the magnetic field of the sense current. The performance of the GMR element is evaluated based on the level of the sense current that causes the magnetization to invert. See, for example, JP 2000-99932A.
By the way, as the recording density increases, a so-called read gap length is narrowed. The read gap length is the distance between the two shields 118a and 118b shown in FIG. 1. In order to narrow the read gap length, the total thickness of the SV film 110 has to be reduced. To this end, the cross-sectional area of the current path of the sense current is decreased. If measurement of inversion of the magnetization of the pinned layer 113 is performed on the SV film 110 with the reduced thickness, the temperature rises due to heat, and the SV film 110 may be broken down. This is because the cross-sectional area of the SV film 110 is reduced, and because the electric current density of the sense current becomes excessive before it reaches the magnetization inversion level. Even though the SV film 110 is not broken down, the antiferromagnetic layer 113 may become paramagnetic, surpassing the Neel point due to the high temperature. Once the antiferromagnetic layer 113 becomes paramagnetic, it cannot pin the magnetization of the pinned layer 113 any longer even after it is cooled down. This prevents accurate detection of the magnetic field produced by the magnetic recording medium 117.